Light gauge continuous sheet metal is produced by rolling mill lines in various thickness and widths. In the case of steel sheet metal, it may be coated at the mill with a thin layer of zinc or zinc alloy in order to provide steel sheet with improved corrosion resistance. After production of the sheet, mill oil is applied in the case of steel sheet and the sheet metal is wound into a coil for shipment to a customer for further processing. Such sheets are used by customers for a number of industrial and automotive applications.
At the customer facility, the metal sheet is unwound and cleaned to remove any mill oil and dirt and to reduce the amount of metal oxide formed on the surface of the metal, after which the metal typically is coated with one or more layers of coating. The coatings usually include at least one primer to provide improved corrosion protection as well as adhesion of subsequent coating layers to the substrate.
One common and very effective method of applying primer to metal substrates is the electrodeposition method in which a primer with an ionic, often cationic, species on the polymer backbone, is deposited on an oppositely charged metal part. The electrocoated parts then are baked to cure the primer. Following the application of the electrocoat primer, other layers of coating such as primer-surfacer can be applied for improved adhesion and smoothness. The final layers of coating applied to the part are those generally seen by the end user. These coatings, in addition to providing protection, such as hardness, weathering protection, and the like to the part, provide a visually attractive finish.
In the production of parts for automobile and other vehicle bodies, sheet metal from the mill, usually galvanized steel, generally is stamped and formed into the desired shape. Prior to this forming step, a lubricating oil typically is applied to the substrate to facilitate the process. The forming oil then must be cleaned from the sheet. Following the cleaning step, the metal usually is pretreated with a phosphate pretreatment. The phosphated metal parts are then assembled into an automobile body with various forms of attachment such as clenching, gluing, and particularly spot welding.
Conventionally, the vehicle body then is primed with a cationic electrodeposition primer. The application of the electrodeposition primer (ED primer) at the automotive manufacturer requires immersion baths large enough to accommodate an auto-body. Such baths require large capital investment and continuous monitoring during production, and also occupy large areas of plant space. Moreover, in some instances, the ED primer may not form a film of sufficient thickness to be effective in confined or partially enclosed areas, for example, those areas where one piece of metal is bent over and clenched to another piece of metal to connect the two pieces of metal. In such a configuration, the ED primer can fail to deposit adequately in the region of the bend, thereby leaving an area of metal relatively unprotected against corrosion. Also, an adequate layer of ED primer may not form in the interior of enclosed parts such as doors.
The process of applying a weldable anticorrosive primer to the metal sheet after cleaning and prior to forming of the metal sheet into an automotive part ensures the presence of an adequate thickness of anticorrosive primer in enclosed or confined areas of vehicle assemblies. Furthermore, application of such a weldable primer to the continuous sheet of metal can be done by roll coating, i.e., a process in which the primer is applied by a roll moving in the same direction, or, more commonly, in the opposite direction, as the moving sheet of metal to be coated. After the weldable primer is applied and dried and/or cured, the continuous sheet of primed metal can be wound into a coil for storage and subsequent shipping. Roll coat application of primer to a continuous strip of metal has the advantage that it is nearly 100% efficient, that is, virtually all of the liquid primer is applied to the metal strip. When cured, the volatiles emitted during the baking process are commonly collected and burned as fuel for the curing oven, leading to low atmospheric emissions. The roll coat application and cure of the weldable primer can be done at a location separate from the vehicle manufacturing plant. Typically application of the weldable primer is conducted at a coil coating application facility, but the weldable primer may even be applied at the steel mill itself. Removal of the priming step from the vehicle manufacturing plant can eliminate the need for the large, expensive ED immersion tanks and, thus, can lead to more efficient use of space and resources in the vehicle plant.
Although use of such coating processes are well known to those practicing the coil-coating art, conventional coil coating primers generally can not be used because the steel sheet, after being cut and formed into parts in a stamping press, is usually assembled into assemblies and vehicle bodies by spot welding. Conventional coil coating primers do not allow sufficient electric current to pass during the spot welding process to cause a weld to form in the metal. If conventional coil coatings are applied very low dry film thickness enough current may pass to form a weld, but at such low thickness corrosion protection is inadequate. The weldable primer of the current invention avoids such limitations by inclusion of electrically conductive pigments as well as anticorrosive pigments to give a weldable formable primer with good corrosion protection. Because the primer is electrically conductive, additional corrosion protection can be realized, if desired, by coating the parts formed from the pre-painted metal with ED primer after they are assembled.
After assembly of the parts formed from the metal sheet coated with weldable primer, the parts may optionally be given an additional phosphate pretreatment, rinsed with water, and dried. The parts can then be coated with any of a variety of top coat compositions known in the art. In co-owned, co-pending U.S. patent application Ser. No.10/025,406, filed Dec. 19, 2001, such assembled parts subsequently are coated with a colored powder coating and, optionally, a clear powder top coating. Powder basecoats and clearcoats are desirable because they are known to provide superior appearance and chip resistance versus liquid coatings; essentially zero VOC versus liquid coatings; and 98 to 99% utilization in most facilities versus 70 to 80% maximum for liquids.
The colored powder basecoat comprises metallic or non-metallic flake pigments. The assembled parts to which the powder basecoat is applied are baked for a period of time sufficient to melt and coalesce the powder coating. If a clear coat is to be subsequently applied to the powder basecoat, the parts are baked for a time and at a temperature sufficient to melt and coalesce the powder basecoat, but insufficient to cure the powder basecoat. The powder basecoat may be used without further coatings, but improved hardness, weathering and UV resistance, and visual appeal will be realized with application of a powder clearcoat. These powder clearcoats provide similar VOC and utilization advantages as those gained with powder basecoats with appearance and durability comparable to liquid clear coats. U.S. Pat. No. 5,407,707 describes the preparation of powder clear coats with excellent physical and chemical properties prepared from epoxy functional copolymers and polycarboxylic acid curing agents.
Upon application of the clearcoat, particularly a powder clearcoat, the coated parts are heated to a temperature and for a time sufficient to co-cure the powder basecoat and clearcoat.
The advantages of the invention are the ability to produce panels and parts, particularly for automotive applications, with striking visual effects, good hardness, and weather and UV resistance by a method that does not require the use of large expensive electrodeposition baths. However, one of the deficiencies of the above-described coating method is that coating defects can form in the cured powder coating due to the escape of air and other gases which are entrapped in the substrate surface and/or in the coating itself. It is believed that these defects arise as the entrapped air and gasses migrate through the basecoat and exit through the clearcoat surface as it is curing.
Thus, it would be desirable to provide a method of powder coating weldable substrates, particularly metallic substrates, which prevents the formation of such defects upon thermal curing of the powder coating system.